Breast cancer is one of the most common malignancies in the United States, affecting one in nine women. Growth factors and their receptors play an essential role in regulating the proliferation of mammary epithelial cells. Abnormalities in the expression of growth factors, receptors, and their signaling pathways contribute to the invasion/progression and maintenance of the malignant phenotype. For example, an overexpression of c-erbB-1/EGF and c-erbB-2/HER2/neu receptors gene products is frequently associated with an aggressive clinical course, decreased disease-free survival, a poor prognosis, an increased metastases in human breast cancer. We have produced a monoclonal antibody against the EGF receptor (mAb 225) which blocks ligand- mediated activation of receptor tyrosine kinase and prevents growth of tumors in both culture and xenografts. Anti-HER2 mAb 4D5 (from Genentech) also demonstrates antitumor activity. Humanized mAbs 225 and 4D5 are currently in phase II and phase III multicentric clinical trials. The focus of this proposal is to establish the mechanism(s) whereby EGF and HER2 receptors participate in the development of invasive phenotypes in breast cancer cells, and to then establish the preclinical efficacy of using humanized mAbs 225 and 4D5 to block breast cancer progression to the more invasive phenotypes, and thus localize metastatic breast cancer. Our working hypothesis is that activation of the EGF and HER2 receptors stimulates the signaling pathways, which in turn, leads to increased cell migration and invasiveness of breast cancer cells and, that these phenotypic changes can be inhibited by anti-receptor blocking antibodies. The rationale behind this proposal is based on our recent observations that blockade of HER2 receptor by mAb 4D5 resulted in inhibiting HRG-mediated stimulation of the PI-3 kinase/PAK pathway, activation of metalloprotease-9 expression, and migration of cancer cells. Similarly, mAb 225 treatment of highly invasive breast cancer cells also resulted in inhibition of in vitro invasiveness. We believe that regulation of EGFR- and HER2-signaling may have significant consequences relating to the invasiveness of breast cancer cells. Invasion leads to metastases and the poor prognosis associated with breast cancer. This proposal aims to: a) delineate the signaling pathways utilized by growth factor(s) to regulate the invasiveness of human breast cancer cells; b) identify the pathway(s) that are sensitive to the anti-invasive function of mAbs 225 and 4D5; and c) develop new in vitro and in vivo models for preclinical efficacy of anti-receptor antibodies in blocking the migration, invasion and growth of breast cancer cells. A unique aspect of this proposal is we will explore the hypothesis that dual receptor blockade by a combination of 225 and 4D5 mAbs may produce enhanced anti-tumor activity by inhibiting proliferation, and thus, tumor invasion and metastasis. Our research may provide a novel rationale for therapy using anti-receptor mAbs 225 or 4D5 in metastatic breast tumors.